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| United States Patent |
5,519,222
|
|
Besett
|
May 21, 1996
|
90 degree parallel path collimators for three head spect cameras
Abstract
A three head SPECT camera system has three detector heads (22a, 22b, 22c)
disposed at 120.degree. intervals. Two of the detector heads have
collimators (40a, 40b) which have a first set of vanes (42a, 42b)
extending in a direction parallel to an axis of rotation (26) but which
are canted 15.degree. relative to a perpendicular, central axis (32a, 32b)
of the respective detector heads. A second set of vanes (46a, 46b) extend
perpendicular to the 15.degree. tipped set of vanes to define a
rectangular grid. When the second set of vanes is perpendicular to the
detector heads, the first detector head is constrained to receive
radiation along a first plurality of parallel rays (44a) and the second
radiation detector head is constrained to receive radiation along a second
plurality of parallel rays (44b). The collimators are mounted to the
detector head such that the first and second parallel rays (44a, 44b) are
perpendicular to each other. In this manner, a full 180.degree. data set
can be collected by rotating the detector heads only 90.degree..
| Inventors:
|
Besett; James L. (Chagrin Falls, OH)
|
| Assignee:
|
Picker International, Inc. (Highland Hts., OH)
|
| Appl. No.:
|
192849 |
| Filed:
|
February 7, 1994 |
| Current U.S. Class: |
250/363.04; 250/363.1 |
| Intern'l Class: |
G01T 001/166 |
| Field of Search: |
250/363.04,363.1
|
References Cited
U.S. Patent Documents
| 5206512 | Apr., 1993 | Iwao | 250/363.
|
| Foreign Patent Documents |
| 250881 | Oct., 1989 | JP | 250/363.
|
| 3285196 | Dec., 1991 | JP | 250/363.
|
Primary Examiner: Fields; Carolyn E.
Attorney, Agent or Firm: Fay, Sharpe, Beall, Fagan, Minnich & McKee
Claims
Having thus described the preferred embodiment, the invention is now
claimed to be:
1. In a SPECT camera which has three radiation detector heads mounted for
rotation about a center of rotation, each of the detector heads having a
central perpendicular axis which (1) is perpendicular to a radiation
receiving face of the detector head and (2) intersects said center of
rotation, THE IMPROVEMENT COMPRISING:
a first collimator mounted to the radiation receiving face of a first of
the detector heads, a second collimator mounted to the radiation receiving
face of a second of the detector heads, the first and second collimators
having vanes that are oriented such that the first collimator passes rays
which are perpendicular to rays passed by the second collimator, the vanes
of the first and second collimators being angled at 15.degree. from the
central perpendicular axes of the first and second detector heads,
respectively; and
a means for rotating the detector heads with the first and second
collimators concurrently with the detector heads receiving radiation.
2. In a SPECT camera which has three radiation detector heads mounted for
rotation about a center of rotation, the detector heads being mounted at
120.degree. intervals around the center of rotation, each detector head
having a radiation sensitive face which converts received radiation into
scintillations of light and electronic circuitry for producing electronic
signals indicative of coordinates on the radiation sensitive face at which
each scintillation occurs and an angular position of the detector head
when each scintillation occurs such that as the detector heads rotate
about the center of rotation, the electronic signals are produced for
radiation received at a multiplicity of angular positions of the detector
heads around the center of rotation for reconstruction into a volumetric
image representation, THE IMPROVEMENT COMPRISING:
a collimator mounted to each of two of the detector heads, the collimators
having vanes that are oriented such that the collimator mounted to one of
the heads passes radiation rays which are perpendicular to radiation rays
passed by the collimator which is mounted to the other camera head, the
vanes of the one collimator being offset by x.degree. from perpendicular
to the radiation sensitive face of the corresponding detector head and the
other collimator having vanes offset by y.degree. from the radiation
receiving face of a other detector head, where
x.degree.+y.degree.=30.degree..
3. A collimator system for a three head SPECT camera system in which the
detector heads are rotatably mounted symmetrically at 120.degree.
intervals around an axis of rotation and in which the detector heads
rotate through a multiplicity of angular positions around the axis of
rotation during radiation detection, the collimator system comprising:
a first plurality of vanes extending parallel to each other and angled at
substantially 15.degree. and a second plurality of vanes extending
parallel to each other and angled at substantially 15.degree., the first
and second pluralities of vanes being mountable to first and second
detector heads of the three head SPECT camera system, respectively,
generally parallel to the axis of rotation and such that projections of
the first and second pluralities of vanes define a square grid.
4. A method of using a SPECT camera system which has first, second, and
third camera heads mounted for rotation about an axis of rotation, each
camera head having a radiation receiving face that is oriented in a plane
parallel to the axis of rotation, the first, second, and third detector
heads being substantially 120.degree. offset from each other around the
axes of rotation, the method comprising:
collimating radiation received by the first and second detector heads such
that the first detector head is constrained to receive radiation
travelling along a first set of parallel rays and the second detector head
is constrained to receive radiation travelling along a second set of
parallel rays, which first and second sets of parallel rays are
perpendicular to each other;
rotating the first, second, and third detector heads substantially
90.degree. around the axis of rotation such that the first and second
detector heads produce a full 180.degree. set of parallel ray data;
reconstructing the full 180.degree. set of parallel ray data into a
tomographic image representation.
5. The method as set forth in claim 4 wherein in the collimating step, the
first and second detector heads are constrained to receive radiation along
parallel paths substantially 15.degree. offset from a perpendicular path
to the radiation receiving faces of the first and second detector heads
respectively.
Description
BACKGROUND OF THE INVENTION
The present invention relates to nuclear medicine. It finds particular
application in conjunction with single photon emission computed tomography
(SPECT) and will be described with particular reference thereto.
In early nuclear or Anger cameras, a patient was injected with a
radioactive dye and a nuclear camera head was positioned stationarily over
the region of interest. The nuclear camera head included a scintillation
crystal which produced a flash or scintillation of light each time it was
struck by radiation emanating from the radioactive dye in the subject. An
array of photomultiplier tubes and associated circuitry produced an output
signal which was indicative of the (x,y) position of each scintillation on
the crystal.
To assure that the radiation causing each scintillation came from a known
path through the patient, a collimator was placed on the patient face of
the scintillation crystal. The collimator typically included a rectangular
grid of lead vanes which assured that each scintillation was produced by
radiation travelling along a path from the patient substantially
perpendicular to the scintillation crystal face.
Other collimators were developed to magnify the region of interest. In a
cone beam collimator, the vanes were tapered such that all the vanes
pointed at a common focal point. Radiation reaching the scintillation
crystal was constrained by the cone beam collimator to radiation
travelling along divergent paths in both the x and y direction such that
the entire scintillation crystal was used to examine a relatively small
region of interest. This magnification improved the resolution in both
planar dimensions. Rather than magnifying in two dimensions, fan beam
collimators were developed which magnified in one dimension. That is, the
vanes were oriented such that the vanes focused the radiation on a focal
line, rather than a focal point.
The data collected by the nuclear camera was analogous to one projection
view or, more accurately, one view of each of a plurality of slices of CT
data. By rotating or indexing the detector head to a multiplicity of
orientations circumferentially around the subject, a full set of CT data
could be collected and reconstructed using conventional CT type
algorithms.
Due to the great weight of the detector head and the lead collimator, a
counterweight was often used to counterbalance the rotating detector head.
Rather than using a passive counterweight, SPECT cameras have also been
built using a second detector head positioned opposite to the first. Two
oppositely disposed detector heads substantially double the data
collection rate. Similarly, positioning three heads at 120.degree.
intervals around the subject, the data collection rate was substantially
tripled. For many examinations, the three heads are positioned
substantially touching such that the patient is substantially surrounded
by radiation receptive surfaces.
CT reconstruction algorithms normally call for 180.degree. of data to
reconstruct a tomographic image. Thus, a single detector head needed to
rotate about 180.degree. around the subject to produce a complete
180.degree. data set. A two-head camera still had to rotate 180.degree.
around the subject to get a full data set, but the full data set was a
360.degree. data set. A three head camera needs to rotate about
120.degree. around the subject to generate a full 360.degree. data set.
By positioning two detector heads at 90.degree. relative to each other, a
full 180.degree. of data can be collected with 90.degree. of rotation.
Even although the two 90.degree. detector heads generate only a
180.degree. data set rather than a 360.degree. data set at a savings of
only 30.degree. of rotation relative to a three head camera, the more
rapid scanning speed is perceived as advantageous for some applications.
The two 90.degree. detector heads have mechanical and data collection
disadvantages.
The present invention provides a new and improved SPECT camera system which
provides the advantages of two 90.degree. detector heads, as well as the
advantages of a three head SPECT system.
SUMMARY OF THE INVENTION
In accordance with the present invention, a collimator assembly is provided
for a three head nuclear camera. Collimators mounted to a first and a
second of the detector heads have vanes angled such that the radiation
reaching the first detector head travels along paths that are
perpendicular to paths travelled by radiation reaching the second detector
head.
In the preferred embodiment, both collimators have their vanes slanted such
that the paths are each offset by 15.degree. from perpendicular to the
detector head.
In accordance with another aspect of the present invention, a collimator
for a three head SPECT camera is provided. The collimator has vanes that
are angled at substantially 15.degree..
In accordance with another aspect of the present invention, a method of
conducting a tomographic examination with a three head SPECT camera is
provided. Radiation approaching first and second detector heads are
collimated such that radiation received by the first detector head travels
along a first set of parallel rays and radiation received by the second
detector head is constrained to travel along a second set of parallel
rays, the first and second sets of parallel rays being perpendicular.
A primary advantage of the present invention is that it enables a full data
set to be collected with only 90.degree. of rotation.
Another advantage of the present invention is that it provides a three head
SPECT camera with the numerous advantages of a three head SPECT camera as
well as the advantages of a two perpendicular head SPECT camera.
Still further advantages of the present invention will become apparent to
those of ordinary skill in the art upon reading and understanding the
following detailed description of the preferred embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention may take form in various components and arrangements of
components, and in various steps and arrangements of steps. The drawings
are only for purposes of illustrating a preferred embodiment and are not
to be construed as limiting the invention.
FIG. 1 is a diagrammatic illustration of a three head SPECT camera in
accordance with the present invention; and,
FIG. 2 is a detailed view in partial section illustrating the geometry of
the radiation reception paths of the SPECT camera of FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
With reference to FIG. 1, a SPECT camera assembly includes a subject couch
or support 10 for holding a subject such as a phantom 12 or a human
patient in an examination region 14.
With continuing reference to FIG. 1 and further reference to FIG. 2, a
gantry 20 supports three gamma camera detector heads 22a, 22b, and 22c at
regular intervals around the examination region 14, i.e. at 120.degree.
intervals. A rotational drive assembly includes a rotating drum or face
plate 24 to which the detector heads are mounted and a drive motor,
bearing, and mechanical linkage assembly (not shown) for selectively
rotating the face plate and detector heads around the examination region.
The face plate 24 includes radially extending, linear tracks or guides
upon which the detector heads are mounted. A linear drive, such as a
motor, rotating drive screw, and a follower selectively slide the detector
heads radially along the guide tracks to position and adjust the distance
between a center of rotation 26 and each of the detector heads. The linear
guides and the drive are illustrated in phantom on head 22c of FIG. 2 but
omitted on the other heads and views for simplicity of illustration.
More specifically, each of the detector heads has a radiation sensitive
face 30a, 30b, 30c, each having a central perpendicular axis 32a, 32b, 32c
which intersects the center of rotation 26. Conventionally, a large sheet
scintillation crystal 34a, 34b, 34c converts received radiation into
flashes or scintillations of light. Photomultiplier tubes 36a, 36b, 36c
packed preferably in a hexagonal close-packed array and associated
resolver circuitry 38a, 38b, 38c convert each event into position
coordinates (x,y) on the radiation receiving face 30a, 30b, 30c and an
energy signal (z) indicative of the energy of each radiation event. An
angular position resolver provides an indication of an angular orientation
of the detector heads around the axis of rotation and a radial position
resolver provides an indication of the distance between the center of
rotation and the radiation sensitive face 30a, 30b, 30c of each detector
head. The (x,y) position signal, the angular orientation signal, the
radial displacement signal, and a knowledge of collimator configuration
provide all the information needed to determine the ray or path travelled
by the radiation for each received radiation event within the coordinate
system of the patient. In this manner, the information can be converted
into a three-dimensional tomographic image representation by a
conventional reconstruction algorithm.
Each of the detector heads carries a collimator 40a, 40b, and 40c. In the
preferred embodiment, the collimators 40a, 40b have a first set of vanes
42a, 42b which extend parallel to the axes of rotation and are sloped at
15.degree. offset from the axes 32a, 32b, respectively. The collimators
40a, 40b are mounted to the respective detector heads 20a, 20b such that
the first sets of vanes slope away from each other. Because the central
axes are 120.degree. offset relative to teach other and each of the
collimators 40a, 40b subtract a 15.degree. offset, the detector heads 20a,
20b are constrained to receive radiation which has travelled along rays
44a, 44b perpendicular to each other, i.e.:
120.degree.-2(15.degree.)=90.degree. (1).
In this manner, two of the three heads of a three head SPECT camera that
are offset by 120.degree. can be effectively turned into the equivalent of
a pair of 90.degree. offset heads. This enables the two detector heads
20a, 20b to detect a full 180.degree. set of data with 90.degree. of
rotation of the rotating face plate or gantry portion 24. Each collimator
also includes a second set of vanes 46a, 46b which are perpendicular to
the first set of vanes such that a grid is defined. When the second set of
vanes are perpendicular to the detector face, parallel ray collimation is
provided. When the second set of vanes are perpendicular to the detector
face adjacent the center and progressively more angled toward the edges,
fan beam collimation is provided.
In the preferred embodiment, the third detector head 22c is not utilized
and is simply shut off. Preferably, the third detector head 22c carries a
collimator 40c to equalize the weight distribution around the axis of
rotation 26. The first and second detector heads 22a, 22b are moved as
close to the axis of rotation 26 as the physical dimensions of the subject
permit; whereas, the third detector head 22c is moved as far from the axis
of rotation 26 as the radial adjustment means permits. Due to the
difference in lever arm which this creates, the third collimator 40c is
preferably lighter than the collimators 40a, 40b in order to compensate
for the different relative radial positioning of the detector heads.
Rather than the collimators 40a, 40b having their vanes 42a, 42b offset at
15.degree. from perpendicular, other angular offsets whose sum is
30.degree. will also enable each detector to detect orthogonal data
permitting the full 180.degree. data set to be collected with 90.degree.
of rotation. As another alternative, the collimator vanes could be offset
at angles which sum to a few degrees less than 30.degree.. This would
enable a full 180.degree. data set to be collected by rotating the
rotating gantry 24 only a few degrees more than 90.degree.. As another
alternative, in some applications it may be appropriate for the third
detector head 22c to be active. If the collimator 40c has vanes tipped at
an angle of substantially 45.degree., the detector head 22c will collect
redundant data with one of camera heads 22a and 22b. When a 180.degree.
scan is to be conducted, a traditional parallel ray collimator 40c in
which the vanes are parallel to the central axis 32c may be utilized. In
this manner, three complete 180.degree. data sets are generated in the
same time that two sets would be generated with a two head camera.
The invention has been described with reference to the preferred
embodiment. Obviously, modifications and alterations will occur to others
upon reading and understanding the preceding detailed description. It is
intended that the invention be construed as including all such
modifications and alterations insofar as they come within the scope of the
appended claims or the equivalents thereof.
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